Most hearing aids include an earmold which is inserted and retained in the canal of the ear of the user. The earmold may contain the electronics of the hearing aid, including the speaker, or may be connected to the electronics, which are outside the canal of the ear, by a tube. The earmold must fit comfortably in the ear and must be formed in a manner that it is readily retained in the canal of the ear. Retention of the earmold in the canal of the ear can be accomplished by friction and/or mechanical locking. Friction is created by radial pressure of the earmold on the wall of the canal. The more pressure, the greater is the retention force. However, friction is also dependent on lubricants between the earmold and the wall of the canal. The presence of cerumen (ear wax), perspiration or water significantly reduces friction retention. Therefore, mechanical locking is the primary means by which most hearing aids are retained in the ear. For mechanical locking, the earmolds are molded to fit the complex shape of the ear canal. These complex interlocking shapes hold the hearing aid in place without relying on friction so that they are not susceptible to the loosening caused by forces which tend to dislodge the hearing aid. However, the making of these complex interlocking shapes is a laborious, inaccurate and time consuming process which often requires the user to make several visits to the audiologist or dispenser before an earmold with a secure fit can be made. This is not only time consuming, but also greatly increases the cost of the hearing aid.
Another problem which arises in hearing aids is acoustic feedback. Acoustic feedback occurs when amplified sound from the hearing aid's speaker enters the microphone and is subsequently re-amplified, resulting in a squealing noise. If the earmold fits well, it attenuates the sound sufficiently to prevent feedback. Therefore, a good fit of the earmold in the canal of the ear is desirable to minimize acoustic feedback. It has been demonstrated that soft earmolds are superior to hard earmolds in the reduction of feedback. However, the earmolds generally used which are specifically shaped to fit a particular ear canal are generally made of a hard material. This provides an earmold which has a longer life and which can be easily removed and reinserted for cleaning and repair.
Two of the more recent attempts at commercializing preformed earmolds are shown in U.S. Pat. No. 4,870,688 to B. Voroba et al., issued Sep. 26, 1989, entitled MASS PRODUCTION AUDITORY CANAL HEARING AID, and U.S. Pat. No. 5,002,151 to R. J. Oliveira et al., issued Mar. 26, 1991, entitled EAR PIECE HAVING DISPOSABLE, COMPRESSIBLE POLYMERIC FOAM SLEEVE. The earmold shown in the patent to Oliveira et al. uses a compressible retard recovery foam that can be compressed and then inserted into a person's ear, and allowed to recover to fill into the canal. This earmold is held in only by friction. Also, the earmold is connected to the electronics by a tube which has the tendency to pull on the earmold frequently and thus dislodge it. For this reason, these devices are limited to short trial periods.
The hearing aid shown in the patent to Voroba et al. uses a soft polymeric material in solid form. The earmold is designed to utilize both friction and mechanical locking. However, the earmold contains the electronics and the weight and cantilever of the hear aid dislodges the earmold over time. To support the cantilever, the Voroba et al. earmold is designed to fill in the canal and the concha. However, making a generic earmold which fits well both the canal and the concha is difficult. Also, the earmold of Voroba et al. is designed for several years of use, requiring that it be made of a harder material.